a) Field of the Invention
The invention is directed to a method and arrangement for the self-calibration of a diode pumped solid state laser (DPSSL), particularly a tunable, diode pumped solid state laser, in which the cavity length is changed for tuning the frequency of the radiation of the laser.
b) Description of the Related Art
It is known from DE 42 42 862 in a solid state laser pumped by laser diodes that the laser-active medium is contacted by a piezoelectric foil which is contacted in turn by an out-coupling mirror. The length of the air gap between the lasing medium and the out-coupling mirror is adapted. Mode selection is achieved by suitable selection of the reflectivity of the mirror. This air gap has a length such that an optional selection of individual laser transitions, Q-switching or tuning of the laser frequency is made possible when voltage is applied to the piezoelectric foil.
DE 40 41 131 A1 and DE 40 42 440 A1 describe a solid state laser with longitudinal single-mode operation whose laser crystal is optically pumped by laser diodes. A piezo-ceramic provided with a high-voltage generator (HV generator) for matching and modulating the wavelength and the fundamental and frequency-doubled radiation is associated with the doubling crystal. The tuning of the fundamental wavelength and second harmonic of the solid state laser is carried out by means of a variable change in the cavity length via the total amplification bandwidth. Different cavity lengths and a change in the longitudinal modes are achieved by means of a variable positioning of the out-coupling mirror.
The laser and the doubling nonlinear crystal are spatially separated from one another and a piezo-ceramic which is controllable by an HV generator is associated with the nonlinear crystal.
It is known from DE 36 43 648 C2 to provide an etalon in the form of an optical plate of small thickness for suppression of the amplitude noise in the resonator cavity in laser-diode pumped solid state lasers with intra-cavity frequency doubling, wherein the resonator cavity is preferably arranged at the location of a beam waist in order to minimize optical losses as well
It is the primary object of the invention to provide a method and an arrangement for calibration in a tunable, diode pumped solid state laser which make it possible to compensate long-duration drift through self-calibration before every scan or as needed and thus, with an expandable tuning range of the laser, to always optimize the output power of the laser.
According to the invention, this object is met in a method for the self-calibration of a tunable, diode pumped solid state laser in which the frequency of the laser radiation of the fundamental frequency and/or the wavelength of the laser radiation of the fundamental frequency and/or doubled frequency is changed comprising the step of changing the optical cavity length by a piezo-actuator or Brewster window over the total amplification bandwidth of the laser-active material and, further including the steps of recording and storing the performance curves during the tuning of an etalon or corresponding optical elements arranged on the cavity, generating or deriving a tuning function for the respective optical element or optical elements from these curves by a microcontroller or computer and adjusting an optimum working point for the optical element or optical elements for maximum suppression of side modes by a digital or analog regulator with the help of a learning curve or learning characteristic.
An arrangement for the self-calibration of the diode-pumped solid state laser which comprises a laser diode as pump light source followed by in-coupling optics, a laser crystal followed by out-coupling optics or a nonlinear, frequency-doubling crystal, wherein the outer surfaces of the laser crystal and doubling crystal or out-coupling mirror have a reflective coating for the laser fundamental frequency and/or for the frequency-doubled radiation and enclose the cavity between them, and further comprises an actuator for varying the cavity length for purposes of tuning the laser is characterized primarily in that an etalon is provided inside the cavity for changing the tuning range and for determining the output power, wherein the etalon is rotatable or swivelable about an axis of rotation which extends at right angles to the optical axis of the laser or is inclined relative to the latter at a small angle.
In order to achieve a multiplication of the frequency of the radiation of the laser, a plurality of suitable nonlinear crystals can be arranged following the laser crystal.
Further details and developments of the invention are disclosed in the additional, dependent claims.
Accordingly, in order to record a learning curve, it is advantageous to tune the etalon or an optical element with increasing amplitude and to correct the deviation from the optimal position at the edge of the tuning range of another optical element. Accordingly, it can also be advantageous when the movement or adjustment of the etalon is adapted to the change in the length of the cavity.
Further, it is advantageous when optimizing an optical element that the latter is itself modulated or another optical element is modulated.
Accordingly, by means of modulating the optical element, a tuning characteristic of the latter or of another optical element is determined and stored.
Further, it is advantageous when the frequency-selective elements of the laser are adjusted between two mode jumps by means of a microcontroller or computer according to the recorded laser characteristic in such a way that side modes are suppressed to a maximum degree.
It is further advantageous when the learning characteristic is adjusted in that the cavity length determining the frequency is tuned as the xe2x80x9cfinestxe2x80x9d frequency-selective element of the laser with increasing amplitude and the mode jumps occurring at the edge of the tuning range are detected (registered) by a suitable measuring instrument or via the output of the laser. The movement of the next coarsest frequency-selective element at the edge of the tuning range is then changed (adapted) until a frequency jump (in the characteristic) no longer occurs. The entire position (movement) of the coarse element is then stored.
Further, according to the method, the power curve or performance curve of the laser is advantageously recorded with a change of the rotational angle "sgr" of the etalon and constant cavity length and with a change in the cavity length and a stationary etalon.
In the arrangement for the self-calibration of the DPSSL, it is advantageous when the etalon is constructed as a transparent disk which is rotatable or swivelable about the axis of rotation and its angle is adjustable by an angular drive.
A stepper motor, known per se, at least one of whose coils is controllable by means of a controlling circuit, can be provided as a drive device. However, a piezo-actuator in operative connection with the etalon directly or with the intermediary of additional elements can also be provided as drive device, wherein it is advantageous that the piezo-actuator comprises a bending element as driving element.
Further, it may be advantageous that only one coil of the stepper motor is controlled. It can also be advantageous when both coils of the stepper motor are controlled, wherein the field vector is modulated to prevent hystereses. The motor can also advantageously be operated in microstep operation.
It has proven advantageous when the rotational axis or shaft of the etalon is arranged so as to be inclined at an angle xcex4 of less than 10xc2x0 in relation to the vertical line to the optical axis of the laser.
The cooling of the moving elements is realized advantageously and by simple techniques by means of an element with good heat conductivity, preferably made of copper or another suitable material. In this respect, it is advantageous when an element is provided for this purpose.
In order to prevent formation of parasitic etalons, the crystals and other optical elements arranged in the cavity are advantageously constructed in a wedge-shaped manner.
Further, a standing wave cavity can be provided in such a way that a secure single-frequency operation is achieved by means of suitable matching of the selectivity of the etalon with the suppression of side modes by spatial hole burning achieved by the arrangement and selection of thickness of the laser crystal.
In another advantageous construction, a piezo-actuator with a stationary etalon is provided for tuning the laser, wherein the free spectral range of the etalon is greater than the amplification bandwidth of the laser crystal and the fineness is selected in such a way that a secure single-frequency operation is ensured in the maximum tuning range. It may also be advantageous that the etalon takes part in the movement to achieve a larger tuning range.